Of the many factors contributing to the obesity epidemic, timing of food consumption is now recognized for its significant contribution to body weight regulation. Disruption of sleep-wake cycles from a predominantly diurnal lifestyle to a nocturnal lifestyle leads to abnormal circadian rhythms and metabolic dysfunction. We have also reported that night eating syndrome, characterized by a delayed pattern of eating, is associated with weight gain and disruption of neuroendocrine rhythms without sleep-wake cycle changes. Studies in night- active rodents have shown that restriction of feeding to the light period leads t profound changes in the circadian rhythms of leptin, insulin, corticosterone, and thyroxine, and a propensity toward obesity, insulin resistance, and hyperlipidemia. However, the metabolic effects of a predominantly daytime versus delayed eating pattern, using an experimental design, have not been well studied in humans. We hypothesize that a delayed pattern of eating, while keeping sleep-wake and activity levels constant, will produce an increase in weight and an abnormal metabolic response in healthy patients. A total of 20 healthy men and women with a BMI of 20 - 27 kg/m2 will be recruited for a randomized cross-over experiment testing two eating conditions. The first condition, daytime eating, would consist of three meals and two snacks to be consumed between 0800 h and 1800 h. The second condition, delayed eating, would consist of three meals and two snacks consumed between 1200 h and 2200 h. Energy and macronutrient content will be comparable between these eating conditions, and sleep-wake period would be held constant between 2300 h and 0700 h. Voluntary exercise will be held constant between conditions. Each participant would be free-living and supplied food from our metabolic kitchen for two months for the first condition, followed by a two-week wash out period eating as they usually would, followed by two months on the second condition (order randomly assigned). Adherence to the protocol will be monitored closely with food logs, pre- and post- weighing of food, use of pictures sent via electronic devices for portion size assessment, actigraphy, and self- reports for sleep-wake and physical activity monitoring. The assessment battery will occur at four points: 1) baseline; 2) after the first 2-mo eating condition; 3) after he 2-wk washout, before starting the second eating condition; and 4) after the second 2-mo eating condition. At each point body composition will be measured with DEXA and indirect calorimetry will be performed. We will measure fasting levels of lipids, triglycerides, and fatty acids. Glucose, insulin, leptin, adiponectin, ghrelin, cortisol, and melatonin will be measured at 0800h, 1200 h, 1600 h, 2000 h, and 0400 h. We will compare the effects of the eating conditions on weight, adiposity, energy metabolism, and hormonal markers. The study will be performed under free-living conditions to increase generalizability. The results obtained from this R21 funded project will establish feasibility for a larger R01 application. The project will benefit frm the expertise of the investigators in eating behaviors, sleep and circadian biology, and neuroendocrinology and metabolism, and has potentially broad clinical implications for promotion of healthy eating habits that could influence weight and metabolic health in the general population.